Stir in the zest, water, & lemon juice; cook over medium heat & contiually stir until the mixture boils. It will take awhile.....just be patient. When the mixture comes to a boil, remove from the heat.

In a small bowl, whisk the eggs together. To temper them, slowly add in some of the hot mixture to the egg yolks {while continually stirring}. Pour the eggs into the hot mixture, stirring them in.

Return to the heat & cook, while stirring, until the mixture is thick {do not boil}; this won't take very much time.

Take filling off the heat and add in the tablespoon of butter, and stir until melted.